1. Field of the Invention
The present invention relates to a reflective Ag alloy film for reflectors, and a reflector provided with the same.
2. Description of the Related Art
A highly reflective Ag film has been used for forming various optical mirrors. Reflectors for automotive lamps, reflectors for lighting devices, and reflecting sheets for liquid crystal display panels are optical mirrors. Optical mirrors are also optical parts for utensils including cameras and video recorders, and semiconductor device fabricating systems including pattern exposure systems.
Reflectors of automotive lamps and lighting devices must be highly heat-resistant because those reflectors are heated at temperatures in the range of 110 to 200° C. by heat generated by a light-emitting element. Reflectors to be used in a high-temperature high-humidity environment must be corrosion resistant. Although reflectors, such as backlight sheets for liquid crystal display panels, and those for optical devices are not exposed to high temperature, those reflectors must have a high reflectivity. Therefore, it is desirable that those reflectors are not coated with a protective film and hence those reflectors are required to have still higher capability to withstand environmental conditions.
The Ag film does not have satisfactory capability to withstand environmental conditions, the ability of the Ag film is deteriorated by moisture. Thus, the Ag film has difficulty in maintaining its function for a long period of use. Therefore, the deterioration of the Ag film is prevented by coating the Ag film with a protective film of a UV-curable resin, an acrylic resin or a ceramic material. The durability of the Ag film coated with the protective film of a resin or a ceramic material is unsatisfactory because the barrier property of the protective film of a resin is unsatisfactory and the protective film of a ceramic material has pinholes and cracks and permits the penetration of moisture. Since cohesion occurs readily in the Ag film when the Ag film is heated to deteriorate the reflectivity of the Ag film, the Ag film is unsuitable for use in a high-temperature environment.
A reflector having improved barrier property and heat resistance proposed in JP-A No. 2000-106017 is provided with a protective film of a resin having improved heat resistance and weather resistance. A technique for improving the corrosion resistance of an Ag alloy disclosed in JP-A No. 2001-226765 adds Pd or Cu to an Ag alloy. A reflector proposed in JP-A No. 9-135096 is provided with an Ag film of an Ag alloy containing 3 at % of one or some of Pb, Cu, Au, Ni, Zn, Cd, Mg and Al. A technique for improving the cohesion resistance of Ag disclosed in JP-A No. 11-231122 adds Pb, Cu, Au, Ni, Pd, Pt, Zn, Cd, Mg or Al to Ag. A technique for improving the cohesion resistance of Ag disclosed in Jpn. Pat. App. No. 13-351572 adds Sc, Y and a rare earth element to Ag. This technique is not intended for application to reflectors.
The reflective thin film of a reflector is required to have a high reflectivity, a high weather resistance and a high heat resistance. However, any metal thin films meeting all those requirements have not been developed and proposed. The conventional Ag thin film having a high reflectivity coheres easily when exposed to moisture containing a halogen or to heat. Therefore, even if the Ag thin film is coated with a protective film, the Ag thin film alters easily starting from parts thereof corresponding to defects in the protective film, and white spots are formed or the Ag thin film is clouded.